The art has recognized for a number of years now the need for upgrading various petroleum feedstocks to remove therefrom various contaminants including sulfur, metals such as nickel, vanadium and iron. For example, it is pointed out in U.S. Pat. No. 4,500,495 that this objective is accomplished by processing the feedstock with a catalyst in the presence of hydrogen. The catalysts employed generally have a solid support such as alumina provided with catalytic metals such as molybdenum or tungsten with either nickel or cobalt. As the catalyst is used, metals and sulfur from the feedstock deposit thereon and/or combine therewith until the activity of the catalyst is impaired to the extent that the desired product quality in the hydrodesulfurized feedstock is no longer achieved. The resulting spent catalyst thus comprises catalytic metals, an inorganic support matrix, e.g., alumina ceramic balls or other shapes, metals removed from the feedstock, sulfur compounds and residual oil. Substantial quantities of spent catalyst on the order of many tons are now being generated annually as poorer grades of feedstock are now being treated. The spent catalyst raises problems on the one side in relation to environmentally acceptable disposition due to its generally oily, sticky physical nature and high sulfur content, and on the other side offers the possibility of reclaiming valuable metals from it.
Although many proposals have been reported in the literature, none of the proposals made to date has been completely acceptable from the standpoint of recovering metals and alumina present in the spent catalyst in marketable form while recovering sulfur as sodium sulfate, and yielding solid, gaseous and/or liquid byproducts which may safely be released to the environment. U.S. Pat. No. 4,495,157 proposed oxygen pressure leaching of spent hydrodesulfurization catalyst using sodium carbonate in the leach slurry. The process and other proposals are operative but are still subject to improvement by way of reducing reagent cost, improving metal recovery, improving filterability and reducing weight and volume of solids generated in the leaching steps and affording lower temperatures and pressures for leaching.
Spent hydrodesulfurization catalysts can vary considerably in composition and in response to treatment. Thus, the materials may contain up to 16% of metal from the group consisting of molybdenum and tungsten, up to 6% titanium, up to 10% silicon, up to 15% vanadium, at least one metal from the group consisting of up to 12% nickel and up to 6% cobalt, up to 12% sulfur, up to 10% coke, up to 20% oil and the balance alumina. Because of the susceptibility of the material to undergo spontaneous combustion, no economical means presently exists for disposal of the material in an acceptable fashion.